Influence of annealing pretreatment in different atmospheres on crystallization quality and UV photosensitivity of gallium oxide films

Due to their high wavelength selectivity and strong anti-interference capability, solar-blind UV photodetectors hold broad and important application prospects in fields like flame detection, missile warnings, and secure communication. Research on solar-blind UV detectors for amorphous Ga2O3 is still in its early stages. The presence of intrinsic defects related to oxygen vacancies significantly affects the photodetection performance of amorphous Ga2O3 materials. This paper focuses on growing high quality amorphous Ga2O3 films on silicon substrates through atomic layer deposition. The study investigates the impact of annealing atmospheres on Ga2O3 films and designs a blind UV detector for Ga2O3. Characterization techniques including atomic force microscopy (AFM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are used for Ga2O3 film analysis. Ga2O3 films exhibit a clear transition from amorphous to polycrystalline after annealing, accompanied by a decrease in oxygen vacancy concentration from 21.26% to 6.54%. As a result, the response time of the annealed detector reduces from 9.32 s to 0.47 s at an external bias of 10 V. This work demonstrates that an appropriate annealing process can yield high-quality Ga2O3 films, and holds potential for advancing high-performance solar blind photodetector (SBPD) development.


Introduction
The earth's atmosphere strongly absorbs and scatters ultraviolet light, primarily due to ozone.As a result, sunlight with wavelengths ranging from 200 to 280 nm has limited penetration of the earth's atmosphere.This specic range of light is commonly referred to as 'solar-blind' or the 'solar-blind band' due to its restricted reach. 1,2The intensity of radiation in this solar blind region is signicantly lower than that in the visible region.Combined with the low natural background, photodetectors designed to operate within this spectral band offer several advantages, including a high signal-to-noise ratio, excellent sensitivity, and a relatively low false alarm rate. 3,46][7] The growth of AlGaN and ZnMgO involves extremely high-temperature requirements and a complex epitaxy process, contributing to a relatively higher cost of lm preparation.In addition, due to the problems of alloy material preparation technology itself, for example, MgZnO with high Mg element ratio is not able to prepare reliable and stable wurtzite structure to meet the requirements of device preparation and light signal detection.The stability of the semiconductor lm is compromised, thereby negatively impacting the feasibility of large-scale lm growth.In comparison to other wide-gap semiconductor materials, Ga 2 O 3 possesses a relatively suitable bandgap width (4.5-4.9 eV), 8 making it well-suited for solar-blind UV detection without the need for additional complex alloying processes.Additionally, its absorption coefficient near the absorption edge reaches 10 5 cm −1 , establishing it as a natural material for solar-blind UV detection with promising practical utility. 9-Ga 2 O 3 continues to serve as the primary structure for gallium oxide-based materials and their solar-blind ultraviolet detectors.However, producing high-quality b-Ga 2 O 3 lms typically demands elevated growth temperatures (>600 °C).The preparation process is complex, and equipment costs are high.10 As exible optoelectronic devices and large-area photovoltaic devices advance, the demand grows for materials with low cost, simplied preparation processes, scalability in production and low preparation temperature.Consequently, researchers are increasingly focusing on the low-temperature growth of amorphous materials.11,12 In recent years, a series of reports have emerged on solar-blind UV photodetectors based on amorphous Ga 2 O 3 , and some devices have exhibited excellent responsiveness characteristics.However, due to the simultaneous presence of a substantial number of oxygen vacancy defects and grain boundaries within amorphous Ga 2 O 3 materials, a comprehensive understanding of the mechanisms through which they inuence the physical properties of the materials and the performance of optoelectronic devices remains elusive.This lack of clarity signicantly constrains the optimal design of both materials and devices.
Recently, an increasing number of scientists have initiated investigations into the growth patterns of amorphous Ga 2 O 3 lms and their corresponding photoelectric properties.Qian et al. synthesized both b-Ga 2 O 3 and amorphous lms using Molecular Beam Epitaxy (MBE). 13XPS analysis reveals that the surface of the magnetron-sputtered lm exhibits roughness, suggesting a potential production of additional surface defect states during the sputtering process.The concentration of oxygen vacancies in the lm was measured, revealing that the oxygen vacancies in the sputtering-deposited lm were twice as numerous as those generated by MBE.
In 2014, Guo et al. 14 examined the effects of different growth temperatures on the structure, surface, and optical properties of Ga 2 O 3 lms using metal-organic deposition on a C-plane sapphire substrate.As the temperature increased, the crystallization properties, grain size, and surface roughness of the Ga 2 O 3 lms also increased.In 2017, Cui et al. 15 from the Institute of Physics, Chinese Academy of Sciences, employed magnetron sputtering to grow Ga 2 O 3 amorphous lms on both quartz and exible substrates at room temperature.Raque et al. 16 grew b-Ga 2 O 3 lms on a C-plane sapphire substrate utilized low-pressure chemical vapor deposition.
In 2020, a Korean team used HVPE method to grow a-Ga 2 O 3 lms with good crystallinity and smooth surface morphology on sapphire substrate. 17The development of the thin lm will promote the further development of a-Ga 2 O 3 in the eld of optoelectronic devices.In 2021, Wang et al. 18 used PLD system to deposit Ga 2 O 3 lm on sapphire substrate, and raised the growth temperature from room temperature to 600 °C.They found that the deposition rate of the lm decreased with the increase of temperature, and the existence of non-lattice oxygen vacancy was conrmed.High quality single crystal Ga 2 O 3 lms with high visible and near-infrared transmittance, large grain size and smooth surface were obtained.In 2023, X. Ji. 19 et al., from Shandong University, prepared amorphous Ga 2 O 3 lms by RF magnetron sputtering technology, and reported a Schottky photodiode with asymmetric electrode.Under a bias voltage of 5 V, the I dark is as low as 6.6 pA, and the light-dark current ratio is as high as 2.3 × 10 6 .Responsiveness up to 1021.8A W −1 ; this result is comparable to or better than the reported high performance b-Ga 2 O 3 Schottky photodiodes and provides a feasible way to achieve large area, low cost, high contrast and high detection sensitivity of solar blind imaging.
In contrast to methods such as MOCVD, ALD enables the growth of thin lms at low temperatures, for example, below 200 °C.This feature is valuable for preparing materials on exible substrates.Moreover, ALD is considered the preferred method for precisely controlling the thickness of micro or nanoscale structures, even at the atomic level, and for conformal covering on substrates with low impurity content and pinhole density.Consequently, when combined with appropriate annealing treatment, the crystal quality of Ga 2 O 3 thin lm materials can be further enhanced.
In this work, 80 nm Ga 2 O 3 lms were grown using the atomic layer deposition (ALD) technique.Subsequently, the lms were annealed for 30 minutes at 800 °C under O 2 , N 2 , and Ar.The surface morphology and chemical composition of each lm were characterized to analyze the effects of different annealing atmospheres on oxygen vacancy concentration and lm quality.Additionally, a Ga 2 O 3 metal-semiconductor-metal (MSM) SBPD was fabricated based on the thin lm.The photodetection properties of the SBPD were analyzed, and the photo-response mechanism of the Ga 2 O 3 thin lms under various annealing conditions was investigated.

Preparation of Ga 2 O 3 lm
Ga 2 O 3 lms were grown on a Si substrate using the BENEQ TFS200 ALD system (BENEQ, Finland).Prior to deposition, the substrate underwent cleaning, followed by rinsing with deionized water and drying within an N 2 atmosphere.During deposition, the precursors employed were trimethylgallium (TMG) for Ga and O 2 plasma for O. TMG was stored in stainless steel bottles at 10 °C.Activation of the O 2 plasma was performed at 200 W.All lm depositions took place at a substrate temperature of 200 °C, under a pressure of ∼3.5 mbar.The thickness of Ga 2 O 3 lms grown in this study is 80 nm.

Sample characterization
The surface morphology of a randomly chosen 500 nm × 500 nm area was examined utilizing AFM (Bruker, Icon) through a non-contact method, in order to characterize the microstructure and morphology of the lms.The sample is tested based on synchrotron grazing-incidence X-ray diffraction (GIXRD) measurements, using X-rays with A wavelength of 0.27 A and a scanning range of 20°-90°.The chemical bonds of the lms were characterized through XPS (SPECS, Berlin, Germany) using a monochromatic aluminum source (hn = 1486.6eV).A narrow scan resolution of 0.1 eV was adopted.All XPS spectra were calibrated using a combined energy of 284.6 eV for C 1s peaks.Photoluminescence performance was assessed with a steady state/transient uorescence spectrometer (FLS980) in the photoemission test section, subjecting each sample to illuminations at wavelengths of 200-500 nm with 1 nm stepsize.The analytical process also encompassed Raman analysis (LabRam HR Evolution; Thermo Fischer DXR).
To examine the I-V characteristics and time response of the detector under both dark and light conditions, we employed the spectral response measuring equipment, model Thermo IS10, in this study.This equipment included a light source, a monochromator, a measurement chamber insulated from external light, a phase-locked amplier, a data measurement source table, and a control computer.Additionally, a 254 nm mercury lamp light source (equipped with a timing control module), a probe station for sample handling, and an Agilent 4155B test instrument for device performance testing and analysis were part of the experimental setup.The timing module controlled the on-off cycles of UV radiation.The Agilent 4155B test instrument was used to apply bias to the device sample on the probe table, facilitating the collection and recording of the device's output current.

Morphology and microstructure
In ALD-prepared amorphous Ga 2 O 3 , a signicant issue persists: the presence of defects, particularly oxygen vacancies.Crystalline oxide semiconductors have lower oxygen vacancy levels compared to amorphous oxide semiconductors, which exhibit higher oxygen vacancy contents. 20Oxygen vacancy defects generally scatter carriers, reducing their mobility and signicantly affecting the material's mechanical properties and thermal conductivity.Annealing holds signicant importance as a technology in the semiconductor industry.Hightemperature annealing supplies sufficient energy for gallium and oxygen atoms to transition to their appropriate lattice positions, allowing them to recongure into the crystal phase. 21,22This promotes the selective growth of the lm, subsequently facilitating the effective release of stress between the lm and substrate and eliminating dislocation defects.However, there is limited research on the inuence of annealing on oxygen vacancy defects in a-Ga 2 O 3 lms and their performance in solar-blind UV detection.In this study's annealing experiment design, three sample groups underwent 30 minutes of constant-temperature annealing in argon, nitrogen, and oxygen atmospheres.The outcomes were then compared with a set of data from unannealed samples.
To analyze the inuence of distinct annealing atmospheres on the surface morphology of b-Ga 2 O 3 lms, we characterized the lms obtained under various annealing atmospheres using AFM.Fig. 1(a)-(d) illustrates the AFM 2D and 3D morphology, as well as the results of surface grain analysis, for Ga 2 O 3 lms annealed in Ar, N 2 , and O 2 atmospheres, respectively.The AFM images were acquired within a scanning area of 500 nm × 500 nm.The 3D AFM representation clearly depicts that the grains in the unannealed sample are relatively small and disordered, whereas the annealed lm surface generally exhibits larger grains. 23,24er annealing, the sample's grain height increases from 2.3 nm to 7 nm, likely due to the transition from an amorphous to a crystalline phase.b (−603) crystallographic planes appear at around 2q = 26°, 39°, and 67°, respectively. 25,26Moreover, these peaks progressively intensify in samples annealed under different atmospheres-specically Ar, N 2 , and O 2 .
From this, it becomes evident that annealing facilitates the provision of energy required for the rearrangement of Ga and O atoms.This, in turn, expedites the migration of atoms within the lm to their appropriate lattice positions, promoting a closer alignment of the lm with the b phase.The process of atomic rearrangement also helps mitigate the internal stresses resulting from lattice mismatch or distortion during the growth of amorphous Ga 2 O 3 , thereby enhancing the lm's quality to a certain extent.Different atmospheres yield different effects on the results.For instance, annealing in an O 2 atmosphere introduces O atoms into the lm's interior, accelerating the transition to the b phase and intensifying the peak value.These observations correspond to AFM images.
The 2D AFM image primarily analyzes the RMS roughness of the lm.The RMS roughness of the unannealed lms measures 0.29 nm, whereas for the lms annealed in Ar, N 2 , and O 2 atmospheres, the RMS roughness values are 0.89 nm, 1.12 nm and 1.23 nm., respectively.As depicted in Fig. 1(f), the surface RMS roughness of Ga 2 O 3 lms increases post the annealing process.This phenomenon can be attributed to the reduction in surface energy within the lm due to annealing, which in turn promotes the polymerization and growth of small grains.Conversely, annealing provides sufficient energy that aids grain polymerization, resulting in increased grain size and corresponding roughness. 27

Composition analysis
To further analyze the concentration, microstructure, chemical valence, and composition of elements in the lm, XPS was employed to measure the chemical bonds of elements within the Ga 2 O 3 lm.The calibration process relied on the C 1s emission line.The complete XPS scan spectrum is presented in Fig. 2(a), which annotates and identies peaks corresponding to all elements.Notably, detailed analysis was carried out on the O 1s and Ga 2p peaks, as depicted in Fig. 2(b) and (c).Following annealing, changes in Ga-O bond concentration and the redistribution of charge around constituent atoms led to variations in peak intensity.This trend exhibited a progressive increase in intensity from the non-annealed state to treatments involving Ar, N 2 , and O 2 .
To analyze the impact of oxygen vacancies on photoelectric properties, we examined the O 1s peaks of each lm.As depicted in Fig. 2(d)-(g), the Ga 2 O 3 sample displays two subpeaks centered at 530.7 and 532.2 eV.The peak at 530.7 eV corresponds to lattice oxygen within the Ga 2 O 3 lm, 28,29 while the peak at 532.2 eV is linked to O 2 ions situated in the oxygen vacancy within the GaO X matrix, commonly known as a Vo-like bond. 30In the unannealed Ga 2 O 3 lm, the ratio of oxygen vacancies to lattice oxygen is 0.27 : 1.However, in the annealed Ga 2 O 3 lm, the oxygen vacancy defect is notably reduced, with the ratio of oxygen vacancies to lattice oxygen in the Ar, N 2 , and O 2 annealed lms being 0.26 : 1, 0.18 : 1, and 0.07 : 1, respectively.
The alteration in the intensity ratio of the two sub-peaks associated with the Ga-O bond indicates a decrease in the concentration of oxygen vacancies following the annealing treatment.Different atmospheres inuence the extent of reduction in oxygen vacancies.In an Ar atmosphere, the inert gas maintains relative stability, causing the impact of lm annealing to closely resemble that of the untreated lm.During N 2 atmosphere annealing, a fraction of N atoms inltrates the lm to occupy vacant oxygen sites. 31Furthermore, N atoms within the lattice capture available O atoms to a certain extent, thus facilitating rearrangement crystallization.Annealing in an O 2 atmosphere results in a signicant inux of O atoms into the lm, effectively replenishing the oxygen vacancies and markedly reducing their presence. 32

Optical testing and photoemission analysis
In order to prepare a high-performance solar-blind UV detector based on the already prepared lm, it is necessary to understand the optical properties of the material itself in advance.Fig. 3(a) illustrates the Raman spectra of Ga 2 O 3 subjected to annealing in different atmospheres.b-Ga 2 O 3 has a monoclinic crystal structure, belonging to the C 2h space group.As a result, the cells of b-Ga 2 O 3 comprise Ga 2 O 6 octahedrons and GaO 4 tetrahedrons. 33Raman activity patterns can be categorized as follows: high-frequency peaks surpassing 600 cm −1 pertain to the stretching and bending of the GaO 4 tetrahedron; midfrequency peaks between 310 and 480 cm −1 correspond to the deformation of the Ga 2 O 6 octahedron; and low-frequency peaks under 200 cm −1 are related to the translation of the tetrahedraloctahedral chain. 34,35Therefore, the standard peaks for b-Ga 2 O 3 are typically characterized by high-frequency peaks above 600 cm −1 and low-frequency peaks below 200 cm −1 .
The peaks within the 400-600 cm −1 range represent the standard peaks of GaO 6 , and those within the 200-400 cm −1 range mostly correspond to the standard peaks of amorphous Ga 2 O 3 .In the 200-400 cm −1 range, the spectral noise uctuations of annealed Ga 2 O 3 lms are signicantly smaller than those of the unannealed samples.This suggests that the annealed samples have transitioned from their amorphous state, resulting in reduced residual strain and a lower dislocation density.Moreover, there is a successive enhancement in the peaks corresponding to Ar, N 2 , and O 2 within the annealed Ga 2 O 3 lms.This enhancement indicates a gradual shi towards a more distinct single b phase structure.This observation is consistent with earlier ndings from AFM and grazeincidence XRD analyses.
Fig. 3(b) depicts the computation results for ultraviolet transmittance and optical band gap.Due to the thickness of the sample, the transmittance of UVA and UVB wavelengths above 280 nm remains only slightly higher than 60%.However, for the UVC band ranging from 200 nm to 280 nm, all lms consistently maintain a relatively high level of transmittance.A certain degree of redshi is evident in the annealed lm when compared with the unannealed lm.Using the transmittance spectrum, the absorption coefficient is determined through the following calculation.
UV/visible light photometers operate based on the principle of the law of light absorption, as described by the formula: 36 where a represents the absorption coefficient, I 0 refers to the steady-state photocurrent, and d is the lm thickness.The calculation method of the absorption coefficient is as follows: 33 As Ga 2 O 3 is a direct band gap semiconductor, its optical band gap and light absorption adhere to the following expression: 36 where h is the Planck constant, n is the incident light frequency, B is a constant, and E g stands for the optical band gap.The tting results are displayed in Fig. 3(b).The value of the optical band gap, E g , is obtained by extending the intercept of the linear segment of the graph along the horizontal axis of hn.Based on the calculated outcomes, the optical band gap value of the unannealed lm measures 4.78 eV.Additionally, the optical band gap values for the lm annealed in Ar, N 2 , and O 2 atmospheres are 4.90 eV, 4.99 eV, and 5.06 eV, respectively. 37,38This demonstrates that annealing induces a band shi in Ga 2 O 3 , resulting in an increased band gap, a phenomenon that is also inuenced by the annealing atmosphere.Fig. 3(d) illustrates the PL spectra of samples subjected to different annealing conditions.A 240 nm excitation light wavelength was employed for these measurements.In the 320-450 nm range, all spectral peaks exhibit consistent linear shapes, with their emission peak positions remaining independent of the sample's morphology.The observable luminescence peak at 350 nm is a result of laser diffraction.Variations in annealing conditions lead to slight differences in the intensity of each spectral peak.When the Ga 2 O 3 material is annealed in argon, the oxygen vacancy defect of the lm does not decrease signicantly, resulting in the emission peak value similar to that of the unannealed lm.However, oxygen atoms in Ga 2 O 3 annealed in a nitrogen atmosphere escape from the sample, resulting in a large number of oxygen vacancy defect donors. 39However, due to the tunneling effect, electrons in the donor are captured by either gallium (Ga) or gallium-oxygen (Ga-O) vacancies, forming trapping excitons and producing emission.The lowest peak value was obtained by annealing in an oxygen atmosphere, which conrmed the lowest oxygen vacancy defect concentration in the lm.
In this paper, the 350 nm excitation light is attributed to vacancy-type defects (V O , V Ga -V O ).Vacancy defects can be categorized into two types: oxygen vacancies (V O ) acting as donor defects, and gallium-oxygen vacancy pairs (V Ga -V O ) acting as acceptor defects. 39,40Both types result in the complex emission of defects.The excitation light near 350 nm is more common for some b-Ga 2 O 3 nanomaterials for blue-purple luminescence bands.Experimental ndings indicate that annealing leads to a reduction in defect concentration and a decline in the peak value of the excitation light.The lowest peak value achieved through annealing in an oxygen atmosphere conrms the lm's lowest concentration of oxygen vacancy defects, aligning with the outcomes observed in the O 1s peak of XPS analysis. 41

Photodetector test
Given the challenges associated with producing high-quality Ptype Ga 2 O 3 lms, the PN junction detectors formed using b-Ga 2 O 3 and other P-type materials consistently exhibit lattice mismatches.This, in turn, leads to heterogeneous epitaxy in the lm, consequently compromising the lm's quality.Moreover, the intricate preparation process required for achieving highquality lms exacerbates this concern, substantially limiting the practical advancement of this technology.In contrast, the MSM structure-based detector has gained signicant attention due to its straightforward fabrication process, minimal junction capacitance, and negligible leakage.To investigate how annealed pairs inuence the photoelectric properties of a-Ga 2 O 3 , we produced metal-semiconductor-metal (MSM) photodetectors using these sample lms.The device features an interdigital electrode with dimensions: a 20 mm width, 20 mm interdigital spacing, and 500 mm length.The conguration comprises 25 pairs of interdigital electrodes, with each Ti/Au electrode measuring approximately 20/50 nm in thickness.
To investigate the impact of annealing conditions on the photoelectric properties of ALD-grown Ga 2 O 3 lms, we prepared the corresponding MSM-SBPD as shown in Fig. 4(e) and (f).Key quality metrics, such as the light-to-dark current ratio (PDCR), response rate (R), and normalized detectivity (D*), 39,40 were used to evaluate the performance of the Ga 2 O 3 photodetector.The photocurrent denotes the device's output current when a bias voltage is applied across its positive and negative electrodes, while it is subjected to a specic wavelength of radiation.Due to the reduction in semiconductor resistivity driven by photoexcitation, the photocurrent is generally signicantly higher than the dark current.
Photogenerated current is dened as the difference between photocurrent and dark current, and its magnitude reects, to a certain extent, the semiconductor materials' photoelectric conversion ability.The formula used for calculating PDCR is as follows: 42 PDCR = (I light − I dark )/I dark (4)   where I dark is dark current and I light is photocurrent.The optical responsivity signies the photoelectric conversion capability of the photoelectric conversion device concerning a given optical signal.When placed under a specic bias voltage, the optical responsivity corresponds to the ratio of the photogenerated current to the incident light power.The magnitude of the optical responsivity indicates the strength of the device's photoelectric conversion ability.The calculation formula is as follows: 43 R = (I light − I dark )/P (5)   where P represents the optical power of the applied light.Generally speaking, the development of light detectors will pursue the increase of these two performance parameters.The signal-to-noise ratio refers to the ratio of signal to noise that is converted into the current output value during detector detection.This parameter holds signicant importance for the detector's detection ability.However, noise generation is a complex process encompassing internal thermal noise of the device, shot noise resulting from material excitation by light, icker noise associated with surface traps, back-bottom noise from the external environment, and other factors.Consequently, the signal-to-noise ratio for a 1 W power light incident on a 1 cm 2 device area denes the normalized detectivity (D*). 44his term essentially quanties the detector's capability to detect the minimum optical signal during normal operation, the calculation is as follows: where R represents the optical response intensity of the device, q signies the amount of electron charge.J d corresponds to the dark current density of the device, which can be calculated by dividing the size of the dark current by the effective area.Evidently, in the context of optical detection, a higher detectivity D* indicates superior performance. 45epeatability is also an important factor for determining the long-term stable dynamic operation of the photodetector.To test the repeatability of the amorphous Ga 2 O 3 ultraviolet detector, we evaluated the time-dependent optical response of the device at a wavelength of 254 nm.During the measurement, a 254 nm light source, with a power of 90 mW cm −2 , was turned on and off every 60 seconds. 46The gure illustrates that all photodetectors exhibit excellent repeatability and stability during operation.
Further analysis of the response time for each detector reveals a higher photocurrent for the same device under a higher bias voltage.Additionally, the response time varies when the lm is switched on and off in different annealing atmospheres.The response time (s) comprises the rise time (s rise ) and the decay time (s decay ).s rise is dened as the time taken for I ph to increase from 10% of the steady state value to 90% of the maximum value, while s decay is dened as the time taken for I ph to decrease from 90% to 10%. 47,48The test curve is tted using a double exponential relationship equation with the expression from ref. 43: where A and B are constants, t stands for time, and s 1 and s 2 are relaxation time constants.The tting outcomes are shown in Fig. 4(e) and (f).Generally, the rise time (s rise ) for each device tends to exceed the decay time (s decay ).This could be attributed to the device's interaction with 254 nm light under applied voltage.The increased generation of electron-hole pairs during this interaction does not lead to an instantaneous change in current with external conditions.This delay is due to the required migration time for carriers.Notably, the photocurrent demonstrates enhancement with higher applied voltage.For instance, the application of a higher bias voltage of 20 V induces greater electron excitation and mobility.This effect accelerates the absorption of photogenerated carrier separation by the electrode, consequently expediting the rise time.
The annealing duration of the lm varies across different atmospheres.The sequence from fastest to slowest annealing is observed in the following order: lm annealed in O 2 , N 2 , and Ar atmospheres, followed by the non-annealed lm.The nonannealed lm demonstrates the longest response time, with a rise time of 9.32 seconds under a 10 V bias.Conversely, the lm annealed in an oxygen atmosphere shows the shortest response time, displaying alterations in s with an increase of 0.45 seconds and a decrease of 0.17 seconds at a 5 V bias.In a semiconductor material, defects may give rise to two types of carrier-effect centers: trap centers and recombination centers.Trap centers can capture a single type of carrier (either an electron or a hole).Typically found at shallow energy levels, they capture electrons during the transition of excited valence band electrons, thereby increasing the device's s (time constant).Additionally, they can also trap electrons descending from the conduction band aer the excitation is removed, further extending the device's s attenuation.Recombination centers, operating through a different mechanism, generally exist at deep energy levels.][51][52] Combined with the prior analysis of lm quality, Fig. 4 illustrates that distinct atmospheres exert a more pronounced inuence on the increased s of the detector subsequent to annealing the device substrate.Hence, the subsequent improvement in epitaxial quality of gallium oxide lms primarily signies a noteworthy reduction in material defects functioning as trap centers, while the density of composite centers experiences marginal change.4][55] The preceding lm's XPS analysis notably demonstrates the decrease in defects like oxygen vacancies, implying that the substrate's annealing pretreatment contributes to diminishing trap centers within the material.This, in turn, extends the duration of s increase in the device.
Fig. 5 depicts the I-V characteristic curves of a gallium oxide solar-blind UV detector under a 10 V bias, while subject to varying substrate annealing conditions and exposed to both 254 nm UV light and complete darkness.The gure reveals that following the annealing pretreatment, the device experiences an increase in dark current alongside a reduction in photocurrent.To comprehend this phenomenon, an analysis of the inuence of annealing on the I-V characteristics of the device was performed.It was determined that annealing treatment contributes to the reduction of oxygen vacancy concentration within the amorphous gallium oxide lm.This decrease in vacancy concentration subsequently leads to diminished intrinsic carrier concentration within the lm, thereby resulting in the observed reduction in the device's dark current. 35,56ccording to previous reports, oxygen vacancies in the absorption layer can trap surplus photogenerated carriers, thereby reducing electron-hole recombination and yielding higher photoconductive gain 57,58 .Hence, the change in I-V characteristics of a-Ga 2 O 3 photodetectors post oxygen annealing primarily stems from the decreased oxygen vacancy concentration.This perspective is also supported by the PDCR analysis.In the scenario of amorphous Ga 2 O 3 , when the device is exposed to ultraviolet light, it induces the photogenerated carriers within the a-Ga 2 O 3 detector.These carriers then migrate towards the electrode under the inuence of the applied electric eld.As these photogenerated holes dri towards the Au/a-Ga 2 O 3 interface, they are captured by the lm's traps, resulting in substantial internal gain. 35,59g. 5 I The XPS analysis has previously conrmed that the unannealed lms exhibit the highest oxygen vacancy content, while lms annealed in an O 2 atmosphere show the lowest content.Consequently, a reduction in the lm's oxygen vacancy concentration leads to a proportional decline in the device's internal gain, subsequently inuencing the device's performance.
As a material for solar-blind UV detection, the UV spectral response characteristic curve also represents essential test data for evaluating the UV photosensitivity of gallium oxide.Fig. 6 illustrates the spectral response characteristic curve of each device within the 200-400 nm range.It is evident that the primary response band of each device is situated around 254 nm.Additionally, there is minimal to no response within the non-solar-blind region beyond 280 nm.This observation underscores the gallium oxide thin lm's exceptional capability for solar-blind UV detection.
The response of the amorphous Ga 2 O 3 detector decreased from 5.84 A W −1 (unannealed) to 0.29 A W −1 (O 2 annealed) aer annealing in different atmospheres.This decrease can be attributed to annealing's role in reducing the number of oxygen vacancies present in the absorption layer.These vacancies have the potential to trap excessive photogenerated carriers, thereby leading to a reduction in detector gain.Consequently, the device's sensitivity to visible light is signicantly diminished.The solar blindness/UV rejection ratio (R 254nm /R 280nm , dened as the ratio of the response at 250 nm and 400 nm) and the solar blindness/visibility ratio (R 254nm /R 400nm , dened as the response at 250 nm and 400 nm) of the a-Ga 2 O 3 photodetector both undergo a notable decline following annealing.
Fig. 6(d) shows the inuence of distinct substrate annealing conditions on the responsiveness and detectivity of the gallium oxide solar-blind UV detector, operating under a calculated voltage of 10 V.The impact of different substrate annealing conditions on the device's responsiveness and its underlying principle closely mirrors that observed in the photocurrent analysis.Nevertheless, variations become evident in terms of normalized detectivity.Notably, solar-blind UV detectors fabricated within Ar and N 2 atmospheres exhibit diminished detectivity when contrasted with their unannealed counterparts.In contrast, the normalized detectivity of UV detectors annealed within oxygen-rich atmospheres surpasses that of their unannealed counterparts.
The main theoretical reason is that, while the enhancement effect of mobility on the device's light and dark current remains the same, the reduction of non-intrinsic excitation resulting from improved crystal quality and diminished defects exerts an inhibitory inuence on photocurrent enhancement.This impact signicantly hampers device responsiveness.However, simultaneously, the increase in noise equivalent power due to elevated dark current signicantly affects the device's normalized detection rate.Similarly, only an appropriate annealing atmosphere (such as the oxygen annealing atmosphere discussed in this paper) can yield a substantial enough increase in material carrier mobility to counterbalance the aforementioned  Paper RSC Advances inhibitory effect, consequently leading to a noteworthy enhancement in the device's normalized detectivity (Table 1).
The inuence mechanism of annealing on photoelectric detection is analyzed from the energy band perspective.It is evident from the gure that the band gap of the amorphous gallium oxide lm is 4.76 eV, whereas for the O 2 -annealed gallium oxide lm, it is 4.98 eV.This implies a slightly smaller band gap width for amorphous Ga 2 O 3 compared to annealed gallium oxide.Due to the long-term periodic destruction of the atomic arrangement and the disorderly uctuation of the lattice potential energy, the conduction band and the valence band extend to the forbidden band to form the tailing of bands, which leads to the small band gap width.XRD results show that the crystallinity of the lm increases with the increase of annealing temperature, resulting in a gradual decrease in the number of unsaturated defects.The reduction of unsaturated defects leads to a decrease in the density of defect states in the band gap, thereby reducing the tailing of bands and thereby increasing the band gap. 60,61Therefore, the increase of the band gap can be attributed to the decrease of the defect state density.
Furthermore, within the amorphous structure, these impurity levels exhibit higher density, consequently extending the upper valence band into the band gap region.For example, the unannealed Ga 2 O 3 exhibits a valence band top at 1.84 eV, whereas annealing Ga 2 O 3 in an O 2 atmosphere shis the valence band top further from the Fermi level to 2.47 eV.Because the top of the unannealed valence band does not deviate from the Fermi level, the band tail effect is stronger, so the band gap of the unannealed Ga 2 O 3 lm is smaller than that of the annealed Ga 2 O 3 lm.A more pronounced tail effect is likely to induce increased intrinsic excitation effects within the amorphous gallium oxide lm under solar-blind ultraviolet excitation.This, in turn, leads to noteworthy alterations in solar-blind ultraviolet photosensitivity, further enhancing photocurrent, device responsiveness, and detection rate.These improvements far exceed those achieved by b-Ga 2 O 3 -based light detectors, aligning consistently with prior R and PDCR test outcomes.

Conclusion
In this work, we prepared amorphous Ga 2 O 3 through ALD growth and annealed it under various atmospheres.Subsequently, we fabricated MSM SBPD.The Ga 2 O 3 -based SBPD without annealing displayed a high PDCR of 1.13 × 10 7 and a D* of 1.32 × 10 4 Jones.Ga 2 O 3 lms annealed in an O 2 atmosphere exhibited a lower oxygen vacancy level (6.54%) and a faster decay time (0.17 s).Furthermore, compared to the unannealed Ga 2 O 3 lms, those annealed in an O 2 atmosphere showed a shi of the valence band away from the Fermi level and an increased band gap.These changes directly led to reduced photocurrent and responsivity.Our results comprehensively assess the impact of different atmospheric annealing approaches on amorphous Ga 2 O 3 .The performance of the lms annealed in Ar atmosphere has no obvious improvement compared with that of the unannealed lms.The properties of the lms annealed in N 2 atmosphere are improved, but the effect is not as good as that in O 2 atmosphere because new oxygen vacancies are created due to the entry of N atoms into the lms.These Ga 2 O 3 SBPDs offer a potential avenue for developing the high light response properties required for future solar-blind detection techniques.

Fig. 3
Fig. 3 (a) Raman spectra of Ga 2 O 3 annealed under different atmospheric conditions.(b) Ultraviolet light transmittance and optical band gap of Ga 2 O 3 annealed in different atmospheres.(c) PL spectra of Ga 2 O 3 films under various annealing conditions.(d) Magnified PL spectral image within the wavelength range of 330-360 nm.

Fig. 4
Fig. 4 Dynamic IV-T results at different bias voltages: (a) unannealed, (b) Ar, (c) N 2 , and (d) O 2 .The (e) s increases and (f) s decays for Ga 2 O 3 detectors in different annealing atmospheres (the illustrations in (e) and (f) are the schematic diagram of the detector structure and the image of the interdigital electrode, respectively).

Fig. 6
Fig. 6 (a) PL spectra of Ga 2 O 3 films under different annealing conditions.(b) Magnified PL spectral image within the 240-280 nm wavelength range.(c) Rejection ratio for each sample.(d) Calculation results for R and D* for each sample.

Fig. 7 (
a)-(d) reveals the band gap width of each sample, calculated based on the XPS results of O 1s sub-peak testing.Fig. 7(e)-(h) presents the valence band spectra for each sample, forming the basis for the band shi diagram shown in Fig. 7(i).